Robot arms have been built in various configurations for many years and have recently come into widespread use in industry. Conventional forms of such arms now in common use have relatively few joints, for example, three to eight, which joints are separated by rigid members. Each of the joints in such conventional arms can either pivot or slide so that the arm can assume many different positions and thus perform useful functions. The joints in conventional arms are generally made from linear or circular bearings such as have been used in machinery for many years.
Conventional robot arms are moved by actuators associated individually with joints in the arm. Generally, the actuators are hydraulic, electric, or pneumatic, moving the joints either in a linear displacement or an angle at each joint. Traditionally, the control systems incorporate feedback loops to attain desired joint positions. That is, if the measured position and desired position do not coincide, control signals are developed to attain the desired position. Rate control is also sometimes used so that the various joints of an arm can be driven at preassigned rates as well as to preassigned positions.
Conventional robot arms are generally made as rigid as possible so that the actual position of the arm corresponds closely to the ideal position as it would be specified by measurements from all of the joints in the arm. Considerable design effort and cost are usually associated with removing unwanted flexibility from an arm. In that regard, some unwanted flexibility occurs in the rigid segments between the joints while other flexibility comes from inaccuracies in the fit of the joints themselves. Such undesired flexibility shows up as an inaccuracy in the actual position of the arm even when each of the joint sensors has attained a proper position or angle. Thus, conventional robot arms require accurate and close-fitting joint bearings along with construction to provide rigidity as well as accurate joint-position sensors.
Conventional jointed robot arms generally have several inherent disadvantages. First, as indicated above, such arms require the use of precision bearings and multiple sensors at considerable expense. Also, because the accuracy of conventional arms is related to the rigidity of the structure, reliable control is difficult when dealing with variable loads on the arm. Furthermore, conventional robot arms frequently incorporate joints which force sharp bends in cables, wires, hoses, and other communication or power-delivery circuits as may be positioned within the arms to service devices along the length of the arm. Such sharp bends are troublesome because the cables passing through them are subjected to repeated sharp flexing.
There have been previous proposals to construct a robot arm of flexible curvature with motion patterns resembling those of an elephant's trunk. A prior U.S. Pat. No. 3,712,481 (Harwood) discloses a mechanical arrangement incorporating rotating wedges proposed to force the arm into a particular curvature. The system requires a multiplicity of bearings with the resulting inherent problems and difficulty of construction and control.
U.S. Pat. No. 3,284,964 (Saito) discloses a system proposing to use a sequence of stacked bellows to accomplish arm curvature. The patent does not disclose any means for distributing the curvature uniformly along the length of a bent segment of the arm. Consequently, the system would be susceptible to control difficulties as explained in somewhat greater detail below.
The structure of U.S. Pat. No. 2,765,930 (Greer et al.) is subject to the same difficulty as that of the Saito patent in that it does not incorporate apparatus for distributing a curvature along a specific length.
U.S. Pat. No. 4,107,948 (Molaug) discloses a mechanical system of flexures that are driven by a single actuator at one end of the arm. The single actuator (rather than a distributed actuator system) fails to attain the desired control for an effective flexible arm.
In addition to the above considerations, each of the structures disclosed in the above patents tends to be mechanically complex and relatively expensive to produce. On the contrary, the system of the present invention provides a flexible robot arm which can be relatively inexpensive to produce and which also can be effectively controlled in a variety of applications.
The system of the present invention incorporates an elongate, skeletal frame member of elemental flexibility which is powered by collective deflection means to seek a particular configuration. The structure further incorporates distribution apparatus for elementally controlling the frame member to accomplish a substantially uniform curve along the length of each section. The sections may be constructed using a skeletal frame formed by a multiplicity of hinged elements or alternatively by a continuous structure as in a double helical form.
In manufacturing the system, a multiplicity of similar preforms produced as by molding or stamping techniques can be assembled to accomplish the flexible robot arm including the deflection and curve distribution means. Several individual sections of curvature may be integrated into a composite arm which is highly repetitive and relatively easy to build and assemble for any of a variety of purposes.